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Tap Drill Size: Maximize Strength and Avoid Broken Taps

Jun 14, 2012   //   by Bob Warfield   //   Beginner, Blog, FeedsSpeeds, Techniques  //  14 Comments

Before you can run a tap through a hole to cut threads, you’ll need to make the hole.  Determining the correct hole size for the tap is important, and there are several factors to consider before just grabbing the nearest twist drill that happens to be a little smaller than the hole (bad idea!).

Good Tap Drill Information is Based on Thread Percentage

First thing is, where do you plan to get your information?   The web is filled with tap drill size charts and many taps arrive with a recommended drill size right on the package.  That’s one approach.  A more comprehensive approach is to use a tap drill size calculator.  As it turns out, there may not be one single size that is best for your tap, despite what the packaging or handy chart may say.  The reason is that most of the time these resources don’t consider thread percentage.

What is a thread percentages?

Imagine the fully formed internal thread.  Each thread rises from valley to peak.  Now suppose you ran a twist drill down the hole and shaved off some of the peaks.  They’re pretty delicate anyway and will wear off quickly.  In fact, they contribute surprisingly little strength.  Kennametal says a 100% thread is only 5% stronger than 75% thread.  But here is the real kicker:

That 100% thread requires 3 times the power to tap!

Why do you care?  because it is the power to tap that breaks taps, for one thing.  Getting 95% of the strength with 1/3 the force on the tap means you’re dramatically less likely to break the tap off in the hole.  Now we all know what happens when we break a tap, right?  Sailors would go running out of the shop if they heard the language we use in that case.  It’s just not a happy thing.

But is it okay to have less than 100% threads?  In fact, many standards bodies insist on it.  For example, American National and Unified thread specifications provide for a maximum of 83 1/3% thread. These specifications also provide a minimum value that varies from approximately 53% to 75%, depending upon the diameter and pitch of thread.

Before getting into how to select a thread percentage, let’s take a look at G-Wizard’s Tap Drill Calculator:

G-Wizard Tap Drill Calculator

In this case, we’re looking at a 1/4-20 thread.  The tap drills are listed in the table and sorted by thread percentage.  Only standard twist drill sizes are shown.  This makes it easy to scan down the list and pick the twist drill for the thread percentage you’re trying to achieve.  Note that cutting taps and form taps have differing requirements, so you’ll want to be sure and pick the correct choice for your tap.

Guidelines for Selecting a Thread Percentage

–  As mentioned, you can get 95% of the strength from a 75% thread, so when strength is the only consideration, this is the one I go for.  Most charts and packaging recommendations are based on a 75% thread.

– Strength can be a function of length of threads.  I don’t start thinking about maximizing strength unless I am limited by the fastening scenario.  For example, if I can only have a very short threaded length that engages (perhaps due to threading a hole in a relatively thin plate), I will realize I need to higher thread percentage.

–  OSG suggests 55-65% threads for most applications.  The force required to drive a tap is way less than for 75%.  This is the range I use most of the time unless I’m really worried about strength.  If the length of the threaded hole is more than about 3x the diameter of the fastener, 55-65% threads are very likely going to be stronger than the fastener itself anyway.

–  For very long thread lengths, minimize the percentage of thread as tapping forces in the bottom of the hole increase with the length of thread.  For very long, read more than 5X the diameter of the fastener.

–  For small diameter fasteners, consider using a lower thread percentage.  Small taps are much more delicate and they’ll appreciate the lower forces required by lower thread percentages.  I typically drop down to the 55% range for what seems to be a small tap.

Still Want to Use a Plain Old Tap Drill Chart?

That’s okay.  We have one of those on our site too.  Go to our Calculator and Charts page and you’ll find a Drill Size and Tapping Chart there.

 

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14 Comments

  • Great resource for drilling in the G-Wizard’s Tap Drill Calculator! I’m the owner of an established machining company in California and this calculator has proved to be very handy on multiple occasions here in my shop! Thanks for the useful information

    • Thanks for the kudos, Phil!

  • I love how you explain things…This now makes perfect sense and I will be applying this post to all my tapping!

    • ERic, thank you for your kind words!

  • […] came in just tells me one size drill and doesn’t mention what thread percentage I will get.  Thread percentage matters when tapping.  G-Wizard is chock full of reference materials and special purpose calculators of all kinds.  It […]

  • […] all about what % threads you want to tap: Tap Drill Size: Avoid Breaking Taps CNCCookbook CNC Blog For your M4 – 0.7, that #30 is an 80% thread, the 3.3mm is a 77% thread, and a #29 is a 60% […]

  • […] Tap Drill Sizes and Thread Percentages:  Read this if you hate breaking taps–I know I do. […]

  • Does it make sense to make a pilot hole with a higher % thread if you are tapping soft material (aluminum) and the length of the threaded hole (3mm) is slightly less than the diameter of the hole (M4)? For example, use a #30 bit (80% thread) rather than following the recommendation provided with the M4 tap of 3.3mm (77%) or a #29 (60%) that OSG appears to suggest?

    • It does, just be aware that the strength of the threads doesn’t improve as quickly as the torque required for tapping.

  • […] than 75% thread. But here is the real kicker: That 100% thread requires 3 times the power to tap! Tap Drill Size: Avoid Breaking Taps CNCCookbook Reply With […]

  • Bob, great post! This is something I never think about, I just blindly go with the drill on my tap & drill chart (75% engagement). I am totally backing down to 55 or 60 for most things now that you bring this up. I don’t even tap w/ my CNC, I use a hand tapper but certain threads (#6-32 and #10-24 are terrible) just won’t tap without the old back and forth method. I bet at 50% they will tap through much more easily and be plenty strong.

  • Alex, I definitely feel much more secure backing off from 75%. I just hate breaking taps. The other trick is to use form taps where possible. They’re strong and they make stronger threads to boot.

  • […] | Taps | End Mills | Drills | Indexable | Composite Tooling | Diamond Coating | Die Products and Tap Drill Size: Avoid Breaking Taps CNCCookbook may be useful/put your mind to ease. That's about all i can say without knowing a bit more about […]

  • Great site. Plenty of helpful info here. I’m sending
    it to a few buddies ans also sharing in delicious.

    And naturally, thanks on your sweat!

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